This invention relates to a dispersion of platelet particles in a polyolefin matrix, in particular, a dispersion prepared by in situ polymerization of the polyolefin.
Nanocomposites, which are dispersions of particles having at least one dimension that is less than 20 nm in a continuous polymer matrix, confer physical property enhancement to the polymer at a much lower particle content than conventional filled glass- or mineral-reinforced polymers. Nanocomposite materials are described in U.S. Pat. Nos. 4,739,007; 4,618,528; 4,528,235; 4,874,728; 4,889,885; 4,810,734; 5,385,776; and 5,578,672; and in WO 93/11190.
Conventionally, these nanocomposites can be synthesized in a two-step process. In the first step, a clay or layered silicate is modified, generally by ion exchange of alkali metal or alkaline earth metal ions (which exist in natural forms of mica-type silicates) with organic cations such as alkyl ammonium silicates or suitably functionalized organosilanes. This modification step renders the normally hydrophilic mica-type silicate organophilic; the concomitant increase in the interlayer spacing between adjacent silicate layers enhances the dispersibility of the modified particles in a polymer matrix. In a second step, a melt-processible polymer and the organophilic particles are compatibilized under high shear to form a polymer with enhanced physical properties. Alternatively, a polymer that is generated in situ can react with functionalized groups in the organophilic particles.
Generally, relatively non-polar polymers such as polyolefins need to be copolymerized or grafted with a polar substituent such as maleic anhydride to facilitate the exfoliation of the multi-layered particles. (See Macromolecules, Vol. 30, p. 6333 (1997).) Without such modifications, the resultant composite will not have enhanced physical properties.
Composites of modified clays in polar-substituted polyolefins as described in the prior art suffer from a number of disadvantages. First, the ion exchange step is costly, time-consuming, and may introduce chemistries that degrade the physical properties of the final composite. Second, the modification of the polymer adds undesirable cost and may produce a more degradable polyolefin by virtue of the addition of oxygen to the polymer. Third, the use of high shear can cause undesirable polymer degradation.
In view of these disadvantages, it would be desirable to prepare polyolefin or polystyrene nanocomposites that do not require pretreatment of the clay or modification of the polymer or high shear processing.
The present invention addresses the aforementioned disadvantages by providing a nanocomposite comprising an in situ prepared polyolefin having dispersed therein nanofiller particles derived from metal layered oxides or metal oxide salts.
In a second aspect, the present invention is a method of preparing a nanocomposite comprising the steps of:
a) dispersing a hydrophilic clay into water to swell the clay;
b) removing the water from the swelled clay to produce an organophilic clay;
c) contacting the organophilic clay with an alkyl aluminoxane in the presence of an inert solvent for the organophilic clay and the alkyl aluminoxane to form a clay/alkyl aluminoxane complex;
d) contacting the complex with a catalyst that promotes olefin polymerization to form a clay/methyl aluminoxane/catalyst complex; and
e) contacting the complex of step (d) with an olefin or a styrene monomer under polymerization conditions to form the nanocomposite.
In a third aspect, the present invention is a method of preparing a nanocomposite which comprises the steps of:
a) dispersing a hydrophilic smectite clay into water to swell the clay;
b) removing the water from the swelled clay by freeze-drying to produce an organophilic clay;
c) contacting the freeze-dried organophilic clay with an excess of methyl aluminoxane in the presence of an inert solvent for the organophilic clay and the methyl aluminoxane to form a clay/methyl aluminoxane complex;
d) removing the solvent and excess methyl aluminoxane from the complex;
e) contacting the complex of step (d) with a metallocene or Ziegler-Natta catalyst in the presence of a non-polar inert solvent to make a clay/methyl aluminoxane/catalyst complex; and
f) contacting the complex of step (e) with a ethylene or propylene under polymerization conditions to form the nanocomposite.
The present invention provides a way of preparing a polyolefin nanocomposite with enhanced physical properties that does not require either an ion exchange step or a polar group modified polymer. Moreover, the polyolefin nanocomposite can be prepared in the absence of shear.
The polyolefin nanocomposite of the present invention is advantageously prepared by first dispersing the multi-layered particles into water under such conditions to swell the particles. These multi-layered particles are preferably smectite clays such as montmorillonite, hectorite, saponite, sauconite, and vermiculite, or layered silicates such as magadiite and kenyaite. Other useful multi-layered particles include illite minerals and layered aluminum or zirconium phosphates, as well as admixtures of the above-mentioned multi-layered particles.
The hydrophilic multi-layered particles are rendered organophilic by first swelling the particles in water, preferably hot deionized water, to form a gel-like slurry. It may be desirable to centrifuge or decant the slurry and discard any precipitate that is formed before water is removed from the slurry.
Water may be removed, for example, by freeze-drying, distillation under reduced pressure, or distillation at ambient pressure, or a combination of these methods. The dehydrated multi-layered material is then advantageously dispersed in a non-polar inert solvent such as pentane, hexane, heptane, octane, and toluene, preferably at a concentration of less than 10 weight percent based on the weight of the solvent and the treated clay.
An alkyl aluminoxane, preferably methyl aluminoxane (MAO), is then mixed with the dispersion of the dehydrated clay, generally as a substantially water-free solution in a non-polar inert solvent to form a MAO/treated clay complex. MAO is preferably added in stoichiometric excess with respect to MAO-active sites in the dehydrated clay. Inasmuch as it is desirable to remove any unreacted MAO, the solution is preferably stripped of solvent and the resultant solid is preferably washed with a solvent for the MAO such as toluene or xylene, to remove substantially all of the unreacted MAO. As used herein, the term xe2x80x9cunreacted MAOxe2x80x9d refers to the MAO that can be removed from the solid complex by solvent washing.
The MAO/dehydrated clay complex that is substantially free of unreacted MAO is advantageously redispersed in a solvent along with a catalyst that promotes polymerization of xcex1-olefins or styrenes. Such catalysts include Ziegler-Natta catalysts and monocyclopentadienyl complexes biscyclopentadienyl complexes, ansa-metallocenes, and indenyl-fluorenyl substituted metallocenes such as those well known in the art. (See U.S. Pat. Nos. 3,645,922; 5,064,802; 5,374,696, and 5,470,993.) The MAO/dehydrated clay complex/catalyst dispersion is then contacted with an a-olefin, an a,o-diolefin, a non-conjugated a-internal double bond diolefin, or a styrene monomer under polymerization conditions to form a polyolefin or a polystyrene nanocomposite. Examples of suitable monomers and/or comonomers include ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, styrene, ethylene-propylenediene monomer, 1,4-hexadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, dicyclopentadiene, ethylidene norbornene, and combinations thereof. The composite contains dispersed nanofiller particles within a polymer matrix.
The polyolefin is preferably substantially free of polar substituents, and more preferably contains no polar substituents. As used herein, the term xe2x80x9cpolar substituentxe2x80x9drefers to any substituent that increases the polarity of the polyolefin or polystyrene. Examples of such substituents include carboxylic acid groups, anhydride groups, silyl groups, and hydroxy groups. As used herein, the term xe2x80x9csubstantial absencexe2x80x9d refers to less than about 1 percent, more preferably less than about 0.1 percent, and most preferably less than about 0.01 percent, based on the weight of the polymer.
The concentration of the dispersed particles in the polymer matrix is preferably not greater than about 10 percent, more preferably not greater than about 5 percent by volume, and preferably not less than about 0.5 percent, more preferably not less than about 1 percent by volume.
The polyolefin nanocomposite of the present invention has improved properties such as yield stress or stress at break over conventional polyolefins. Such improvements result in a composite material that can be used to make molded parts.
The following examples are for illustrative purposes only and is not intended to limit the scope of this invention.